knock off vs take a rain check

knock off

verb
  • To halt one's work or other activity. 

  • To rob. 

  • To defeat. 

  • To have sex with. 

  • To remove, as a discount or estimate. 

  • To make a copy of, as of a design. 

  • To assign (an item) to a bidder at an auction, indicated by knocking on the counter. 

  • To remove by hitting (something, someone) 

  • To kill. 

  • To accomplish hastily. 

take a rain check

verb
  • In social interactions, to request a deferral of an invitation. To "take a rain check" is a polite way to turn down an engagement, usually with the implication that one is simply postponing it and another time might be acceptable. 

  • To request or accept a rain check (an agreement from a merchant to honor a special offer, temporarily unavailable, after the expiration date). 

How often have the words knock off and take a rain check occurred in a corpus of books? (source: Google Ngram Viewer )